I went and got a new battery for my wife's Trailblazer. Prior to installing it the A/C was working fine.

After i put the battery in the A/c stopped blowing thru the dash vents. It blows fine at the feet and windshield vents but not the dash.

I turn the switch to all the positions and it seems to work on all but the dash vent setting and the dash/feet settings (still blows at the feet only).

Any of you have a clue, what might be the problem.

I just replaced the battery, and the Exact same thing happened to my trailblazer. I am currently trying to find a solution to this problem. I am hoping that somebody has a quick easy solution.

Auto system or manual?

Auto system or manual?

Auto,

I think i found the problem, on another forum i saw a post about the blower register causing problems like this. But most of the time it would be an issue with certain speeds not working.

They suggested that I disconnect the battery for about 15-30 min and see if the system resets. if not the that switch might need replacing.

Don't do that, remove the ac fuse, or control fuse for 5 minutes

what was the solution did that work?

Don't do that, remove the ac fuse, or control fuse for 5 minutes

Tried this and it is still the same. I am not sure if the register is the issue or not. Anyone have any ideas

My father has an '02 blazer LTZ and this, too, happened to him. He was working on replacing a coil pack that went out and disconnected the negative terminal to the battery. That was all it took to trip up the system and when he reconnected it he faced the new issue with the A/C vent stuck on the floor setting. In fact, it makes about 30 thumps every time he turns the ignition on.

IMHO, I would say its the servo motor that controls the directional valve. I have a 97 Monte with a similar issue and it is stuck on defrost. Never bothered to fix though. Anyone UPDATES???

Found the solution here: http://chevroletforum.com/forum/archive/index.php/t-8559.html

They confirmed here that it is most likely the servo actuator for the air directional flap. Hope this helps someone else. I will post an update of my issue when I get to look at it. THANKS

Okay, I have verified it is the actuator for the air conditioner. This is a bigger task than the link above has outlined and requires removal of the dash board to get to the housing screws. The servo motor was available at autozone and didn't come with anything else for 130.00. The dealer had one with extra parts for 72.00. We haven't fixed it yet but if you want your vents to stay on instead of defaulting to defrost then you can unplug the connector to the servo motor while it clicks to lock it into place.